Forming a protective coat of one metal on a second metallic substrate workpiece is well known.
Electroplating or electrogalvanizing is a known method for forming a protective coating of one metal upon a metallic workpiece. Generally in galvanizing a steel workpiece forms a cathode in an electroplating cell containing an electroplating solution which carries metal ions. An anode, which in galvanizing is usually zinc, is positioned in a spaced apart relationship with the workpiece. Upon application of direct current to the cell, zinc ions in the electroplating solution are plated onto the cathodic workpiece as elemental metal. Simultaneously zinc from the anode undergoes electrochemical dissolution to the metal ion, thus replenishing the zinc ions in the electroplating solution.
There are problems in electroplating, especially with the use of so called consumable anodes. For economic and quality reasons it is preferred that no more than a minimum required thickness of the coating metal be deposited on the metallic workpiece. Deviation can cause "weak spots" in the coating.
The achievement of a uniform coating on the workpiece depends upon a number of factors. One of the most significant is uniformity of current density across the workpiece plating surface. Metal ion concentration proximate the workpiece plating surface as well as uniformity of metal ion concentration in a given volume of electroplating solution are also significant factors.
As the anode is consumed the spacing between the anode and the workpiece changes causing changes in the anode to workpiece spacing and attendant changes in the current density. In addition, the consumable anode is not homogeneous and undergoes electrochemical dissolution, unevenly. This uneven dissolution or "contouring" of the anode surface produces non-uniform change of the distance between the workpiece and the anode, with the result that the current desnity changes are uneven across the workpiece plating surface. This phenomenon can cause variations in the thickness of the coating. Further, as the current density changes unevenly across the anode surface it causes an even greater discontinuity in the dissolution.
It has been suggested that non-consumable anodes, i.e. those which are electrically conductive but substantially chemically inert in the electroplating cell, be utilized in order to maintain a constant anode to workpiece spacing across the workpiece plating surface. As with a consumable anode, a potential difference is maintained between the non-consumable anode and the workpiece such that metal ions in the electroplating solution are plated onto the workpiece as elemental metal. As the metal ions are reduced to their metallic state to plate the workpiece, the electroplating solution adjacent or proximate the workpiece becomes depleted of the metal ions. Unless the solution is continually replenished and agitated at the workpiece plating surface so that the ion concentration is maintained, plating will not be consistent. Since a non-consumable anode does not replenish plating ions, as does a consumable anode, the metal ions in the plating solution are replenished from a source remote from the cell.
The use of a non-consumable anode is shown in our co-worker's pending application, SN 217,806 filed Dec. 18, 1980 under the title, "Apparatus and Method for Plating One or Both Sides of metallic Strip" now U.S. Pat. No. 4,367,125 which is incorporated by reference. The anode disclosed in that application includes a series of apertures through which plating solution flows to contact a strip to be plated.
In order to obtain high electrical efficiency and to maintain quality control, the gap or distance between the workpiece plating surface and the anode should be minimized. The effect of minimizing the gap is to limit the volume of plating solution and metal ions near the workpiece and available for plating the workpiece. Thus, this close spacing requirement further limits the ability to achieve efficient, continuous, electroplating using non-consumable anodes. The problems of replenishing or maintaining the plating ion concentration has inhibited performance of many prior non-consumable anode systems with the result that they have not enjoyed abundant commercial success.
For various reasons in recent times applications have developed where it is desirable to plate only one surface of a workpiece or apply coatings of different thicknesses on opposed strip surfaces. For example, single surface coated steel materials are used or proposed for wall panels, buildings, and automotive components. Automotive steel rocker panels for example, frequently are desirably heavily electrogalvanized on their internal surfaces to inhibit corrosive attack while external surfaces desirably are smooth and uncoated or thinly coated so one can produce a high quality appearance in the auto's finish.
One technique for such one side plating is disclosed in the referenced application. Another technique seeks to use a conventional electrolytic strip plating line modified to maintain the level of plating solution at a level where it contacts only the lower surface of the workpiece being plated. A further technique for single side plating masks one surface while plating the other. In this method the workpiece is reeved over rollers that are partially immersed in a plating bath. Such rollers function to mask the workpiece surface which they contact as the opposite surface is plated.
It has also been proposed to use multiple electrodes in an electroplating cell. Generally, attempts at using multiple anode systems and especially non-consumable anode systems have resulted in bi-polar plating action. Bi-polar plating action occurs when the electrical potential between anodes causes deposition of metal on the surface having the lower potential. The lower potential anode acts as a cathode in a cell with the higher potential anode, resulting in a decrease in plating efficiency.
If these and other problems of multiple anode plating could be overcome, it would be advantageous to have a system wherein the advantages of the non-consumable and consumable anodes could be achieved simultaneously.